docs: def seed(context, data):
      """Initialize a crawler with a set of seed URLs.
  
      The URLs are given as a list or single value to the ``urls`` parameter.
  
      If this is called as a second stage in a crawler, the URL will be formatted
      against the supplied ``data`` values, e.g.:
  
          https://crawl.site/entries/%(number)s.html
      """
      for key in ('url', 'urls'):
          for url in ensure_list(context.params.get(key)):
              url = url % data
              context.emit(data={'url': url})